1. Field of the Invention
The present invention relates to a cutting tool or insert that has an integrally mounted sensor to produce a signal indicating tool condition and operation parameters.
2. Description of the Prior Art
Unmanned machining centers are rapidly being developed for factory use, and at present numerical control machines of various types are inherently compatible with unmanned manufacture. Automatic material inspection systems have been advanced as well, but a principal difficulty in realization of automated, unattended operation on an around the clock basis is the lack of effective and reliable sensors to monitor the unmanned machining centers production systems under computer control. Accurate sensing of the condition of the cutting tool is critical to operation and so far has been unattainable. Sensing so that if the tool breaks or cracks, it is immediately noted is important. Also, if it wears beyond acceptable limits, or otherwise deviates from desired parameters, a signal is needed to give an indication so that change of tools can be accomplished quickly. Worn tools, dull tools, or fractured tools result in manufacture of products outside specifications, and also they may damage the part that is being made or cause damage to the machine tool itself, which invariably leads to increased manufacturing costs and loss of manufacturing capacity. Thus, one important feature of the present invention is to prevent such losses and ensure satisfactory operation of unmanned machining centers.
In the prior art, adaptive control systems have been advanced which monitor a manufacturing process as it is being performed in machining systems so that the processing conditions can be changed in order to make the machine more productive. Adaptive control systems which operate by sensing machine spindle drive motor current are already commercially available. There are some force sensing and feedback control systems also on the market, but generally speaking they are not regarded as satisfactory as shown in the prior art references cited below. Thus, the present invention also is designed to fill the existing need for an adaptive control that is accurate, reliable, and "on-line."
Where the monitored machine is in a production line, failure to sense damage to a single tool can cause losses that are much greater than in individual machining centers, because an entire line may be affected. Therefore, the present invention will find wide acceptance in these types of operations, as well.
In the prior art, studies have been made on the dynamics of formation of chips during machining, and various methods have been arrived at for measuring machining forces.
For example, in an article entitled "A Critical Review of Sensors for Unmanned Machining," by Tlusty, D.J., and Andrews, G.C. (Annals of the C.I.R.P., 32-2, 1983), several different types of sensors are examined on the basis of operating reliability for unmanned machining. This article states that the crucial problem that must be overcome in order to achieve the full potential of unmanned machining centers is the development of reliable and effective sensors for monitoring machine operation, for ensuring efficient metal removal rates, and for taking corrective action in the event of accidents or breakages. The article provides a survey of the then available sensors, all of which have serious limitations insofar as reliability and direct response, particularly because of mechanical filtering of signals on the tool through interfaces of the tool and tool support.
A typical dynamometer sensor that mounts a cutting tool is described in an article entitled "Identification of Chip Formation Mechanism Through Acoustic Emission Measurements," by Kunio Uehara and Yuichi Kanda (Annals of C.I.R.P, 33-1, 1984). The dynamometer or platform for mounting the tool holder is satisfactory to sense equilibrium phenomena, such as average cutting forces, using DC level measurements. However, the AC components or the high frequency forces are filtered away by the interfaces between the cutting tool or insert and its tool holder, the tool holder and the dynamometer, and by the dynamics of the instrumentation system itself. As a consequence, dynamometers are unable to sense phenomena occurring at much over 1 kHz.
The mounting of an accelerometer onto a tool holder and using the assembly in connection with a force sensor is described in an article entitled "Measurements of the Segmentation Frequency in the Chip Formation Process," by B. Lindberg and B. Lindstrom (Annals of C.I.R.P., 33-1, 1983). While accelerometers can sense very high frequencies, the ability to sense phenomena occurring at the tool cutting edge at much over 5-10 kHZ is limited because the cutting tools or inserts are clamped mechanically in the pocket of a tool holder and the transmissibility of signals across the tool holder and insert interface is low. Thus, high frequency phenomena occurring right at the cutting edge of the tool are not efficiently communicated to the tool holders, and accelerometers installed on the tool holders are therefore unable to sense high frequency forces or vibrations experienced by the cutting tool or insert.
Under normal commercial cutting conditions, the crystalline grain passing frequency at the cutting edge is in the MHZ range. The crystalline grain passing frequency and other dynamic phenomena such as chip segmentation and the like act substantially as background noise generators. The dynamic behavior of the cutting system reflects this noise generation, and as a result it oscillates with modes and frequencies compatible with its structure and the impressed excitation frequencies. Motion of the tool holder and the tool post will result in the work piece having waviness components on its surface finish that follows this motion. In existing systems using existing sensors, these motions are filtered and cannot be sensed either for process monitoring or for process control.